Methods and apparatuses of detecting the positions of mobiles such as a mobile robot and coordinate input devices such as an electronic pen and the like by using an ultrasonic wave and measuring the propagation time thereof have been proposed (see, for example, Patent Documents 1 to 3). In the position detection method utilizing the propagation time of ultrasonic wave, a mobile transmits an ultrasonic wave and an infrared ray at the same time at regular intervals and a receiver receives them. FIG. 7 shows an example of the signal detected by the receiver. In this example, the ultrasonic wave transmitted from the mobile is a signal of burst waveform and the signal of the same waveform is transmitted for every transmitting intervals. Since there is the difference in speed between the ultrasonic wave and the infrared ray, a time lag (for example, t1 and t2) occurs at the time of the receipt of them. The receiver first receives the infrared ray and then receives the ultrasonic wave that arrives later according to the propagation distance. Hence, the period of time between the receipt of the infrared ray and the receipt of the ultrasonic wave is measured as the propagation time of ultrasonic wave, and the position of the mobile is specified utilizing the propagation time.
According to this method, in the case where plural mobiles are used simultaneously, the receiver cannot identify each of the mobiles. Therefore, identification of the ultrasonic wave corresponding to a certain infrared ray signal is difficult and the coordinate cannot be detected normally. Further, in the state where plural mobiles are activated simultaneously, there is a possibility that the receiver receives ultrasonic waves transmitted from different mobiles at the same time. Since the ultrasonic wave transmitting signal is a signal of burst waveform, it is very difficult to separate the respective mixed ultrasonic waves, and it is difficult to detect the arrival time of each ultrasonic wave. Therefore, the position of the mobile cannot be detected correctly.
Hence, in order to identify each of the plural mobiles and detect the position thereof, a pen input device of simultaneously activating pens by transmitting ultrasonic waves in a time-divided manner has been proposed (see, for example, Patent Document 4). In this pen input device, by giving, with a stationary body, timing signals to pens at the time when the pens are inserted into pen holders, the clock at the stationary body side and the clock at the pen side are synchronized, and the time-division is achieved by dividing the timing signals to be given for each of the pens.
Further, as another example of identifying each of the plural mobiles and detecting the position thereof, a method of transmitting the infrared ray or the like after an infrared ray or the like has been applied with an unique identification signal has been proposed (see, for example, Patent Documents 5 and 6). In this position detection method, it is configured that a stationary body (receiver) transmits an electromagnetic wave signal such as an infrared ray signal including ID information, and the mobile transmits an ultrasonic wave only in the case where it receives the electromagnetic wave signal corresponding to its own ID. Here, the interval of transmitting the electromagnetic wave signal including the ID information is set longer than the period of time in which the ultrasonic wave moves in a maximum range of inputting coordinate. As shown in FIG. 8, if it is assumed that the number of mobiles to be used is n and the interval for measuring the coordinate is T, the time assigned to one of the mobiles T/n should be decided based on the relationship with the moving range. In the case where the handwriting written by an electronic pen is reproduced, T cannot be set long needlessly. For example, in the case where the drawing range is A4-size and T is 10 ms, since a maximum propagation distance of the ultrasonic wave is 350 mm and the propagation time of the ultrasonic wave is about 1 ms, when T/n is set as 2 ms in consideration of margin for not interrupting the time frames of other IDs, up to five mobiles can be used simultaneously. On the other hand, under the assumption that the electronic pen is used at a projected screen by a projector, since the furthermost point reaches 2 m and the propagation time is about 7 ms, two mobiles can be used simultaneously in consideration of margin under the condition of T=20 ms.